Submitted to: Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/23/2009
Publication Date: 6/25/2009
Citation: Cummings, S.P., Gyaneshwar, P., Vinuesa, P., Farruggia, F.T., Andrews, M., Humphry, D., Elliott, G.N., Nelson, A., Orr, C., Pettitt, D., Shah, G.R., Santos, S.R., Krishnan, H.B., Odee, D., Moreira, F., Sprent, J.I., Young, P.W., James, E.K. 2009. Nodulation of Sesbania Species by Rhizobium (Agrobacterium) Strain IRBG74 and Other Rhizobia. Environmental Microbiology. 11:2510-2525. Interpretive Summary: Nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by legumes for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. Sesbania is a genus of approximately 60 species of tropical legume of which 40 have so far been reported to nodulate. Many species occur naturally in wet or flooded soils and these have considerable potential as green manure in wetland rice production due to their ability to fix large quantities of N2. The present study concerns a Rhizobium strain IRBG74 that has been used to promote the growth of rice, as one of a group of plant growth-promoting rhizobacteria. In this study, the symbiotic properties (i.e. host range and nodA sequences) of IRBG74 and the other Sesbania symbionts are discussed in the context of a molecular phylogeny of Sesbania. Information provided in this basic study will lead to a better understanding of biological nitrogen fixation in green manure crops. Further research in this area will help farmers to increase crop yield through less chemical fertilizer input and more recycling of plant nutrients.
Technical Abstract: Concatenated sequence analysis with 16S rRNA, rpoB and fusA genes identified a strain (IRBG74) isolated from root nodules of the aquatic legume Sesbania cannabina as a close relative of the plant pathogen Rhizobium radiobacter (syn. Agrobacterium tumefaciens). However, DNA:DNA hybridisation with R. radiobacter, R. rubi, R. vitis and R. huautlense gave only 44, 5, 8, and 8% similarity, respectively, suggesting that IRBG74 is potentially a new species. Additionally, it contained no vir genes and lacked tumour-forming ability, but harboured a sym-plasmid containing nifH and nodA genes similar to those in other Sesbania symbionts. Indeed, IRBG74 effectively nodulated S. cannabina and 7 other Sesbania spp. that nodulate with Ensifer (Sinorhizobium)/Rhizobium strains with similar nodA genes to IRBG74, but not species that nodulate with Azorhizobium or Mesorhizobium. Light and electron microscopy revealed that IRBG74 infected Sesbania spp. via lateral root junctions under flooded conditions, but via root hairs under non-flooded conditions. Thus, IRBG74 is the first confirmed legume-nodulating symbiont from the Rhizobium (Agrobacterium) clade. Cross-inoculation studies with various Sesbania symbionts showed that S. cannabina could form fully effective symbioses with strains in the genera Rhizobium and Ensifer, only ineffective ones with Azorhizobium strains, and either partially effective (M. huakii) or ineffective (M. plurifarium) symbioses with Mesorhizobium. These data are discussed in terms of the molecular phylogeny of Sesbania and its symbionts.